Well, the first thing to note is that every person's hair is different (but similar). Therefore there is a range of temperatures to consider and what might be low heat for you may be high heat for someone else. I would suggest that if you are straightening, you should attempt to use the lowest possible heat for the shortest possible time.
Human hair has a similar temperature profile to wool as they are both composed of the protein keratin (Thermochimica Acta, pg5-9, 1999). Wool was more comprehensively reported and therefore I'll quote those figures (Adv Exp Med Biol. p 329-344, 1977)
Stage 1 : At and below 150 degrees C (302F)
Loosely bound water and tightly bound water is lost from the hair
Stage 2: At 160-175 degrees C (320 -347F)
Hair undergoes a glass transition phase (meaning the hair begins to flow as hot glass would). Plastic deformation is possible (meaning in a 'normal' hydrated state, hair is elastic and can stretch and return back to its original length. Hair does also have temporary plasticity which is why styles like roller sets, twist outs/knot outs can occur. However at this heating stage, the plasticity is NOT temporary, the hair may return to a 'normal' looking state but it will not be exactly the same)
Stage 3: Between 215 and 235 degrees C ( 419-455F)
The keratin in hair has a natural twist to it knows as an alpha helix (α-helix). This twist is present in all keratin, whether straight hair or curly hair. At this stage the helices of the hair protein melt (not reversible).
Therefore this research suggests that under 150 degrees C (302F) is probably the best for straightening. Yes you will have to trade moisture for straightening. If your hair doesn't get straight, then it probably isn't meant to be, don't turn up the heat! My hair for example has never been straight, even when I relaxed, the kinks like me and I love them :)
Does your flat iron have a temperature regulator? Do you know how hot it actually is?